A conventional piston arrangement used, for example, in internal combustion engines comprises a piston head moveable within a cylinder coupled to a crankshaft by a con-rod. In an alternative piston arrangement a piston head may be coupled to a track having a cam surface, the piston head being provided with one or more cam followers which run along the track to control movement of the piston head. The present invention relates to improvements in the power transfer mechanism coupling a piston head to a track in such an alternative piston arrangement.